java - 错误 : java. lang.NullPointerException 以及更多来源未知的错误

Question

我在修复代码时遇到困难。除了运行小程序时遇到的异常数量之外，它编译得很好。我不断收到的错误消息是:

线程“AWT-EventQueue-0”中出现异常 java.lang.NullPointerException 在 WholePanel$Canvas.paintComponent(WholePanel.java:82)

以及众多其他来源未知的内容。我查看了网站上的各种 NullPointerException 问题，但没有一个有太大帮助。

这是我的代码:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*; // To use listener interfaces
import java.util.ArrayList;

public class WholePanel extends JPanel
{
   private ArrayList <Rect> rectList;
   private ArrayList <Rect> newList;
   private boolean flag;
   private Color currentColor;
   private Canvas canvas;
   private JComboBox colorList;
   private JButton erase;
   private JButton undo;
   private JPanel buttonPanel;
   private JPanel controlPanel;
   private JSplitPane sp;

   public WholePanel()
    {
      //Here we use black to draw a rectangle
      currentColor = Color.black;
      String colors[] = {"black", "red", "blue", "green", "orange"};
      JComboBox<String> colorList = new JComboBox<String>(colors);
      ColorListener colorListener = new ColorListener();
      colorList.addActionListener(colorListener);

      undo = new JButton("Undo");
      undo.addActionListener(new ButtonListener());

      erase = new JButton("Erase");
      erase.addActionListener(new ButtonListener());

      buttonPanel = new JPanel(new GridLayout(1,2));
      buttonPanel.add(undo);
      buttonPanel.add(erase);

      controlPanel = new JPanel();
      controlPanel.setLayout(new BoxLayout(controlPanel, BoxLayout.X_AXIS));
      controlPanel.add(colorList);
      controlPanel.add(buttonPanel);

      canvas = new Canvas();

      JSplitPane sp = new JSplitPane(JSplitPane.VERTICAL_SPLIT, controlPanel, canvas);
      setLayout(new BorderLayout());
      add(sp);

      //To be completed

     }


    private class Canvas extends JPanel
     {
        //This method needs to be defined to draw in this panel

        private Point startingPoint, endingPoint, movingPoint;
     private Rect rectangle;
     private int x, y;

     public Canvas()
     {
      PointListener pointListener = new PointListener();
      this.addMouseListener(pointListener);
      this.addMouseMotionListener(pointListener);
     }

      public void paintComponent(Graphics page)
       {
         super.paintComponent(page);

         setBackground(Color.white);

         for(int i = 0; i < rectList.size(); i++)
         {
          rectList.get(i).draw(page);
         }

         if(endingPoint != null)
         {
          startingPoint = null;
          endingPoint = null;
          movingPoint = null;
         }

         else if(movingPoint != null && x >= 0 && y >= 0)
         {
          page.setColor(currentColor);
          page.drawRect(startingPoint.x, startingPoint.y, x, y);
         }

        //To be filled
       }
     } //End of Canvas class

    private class PointListener implements MouseListener, MouseMotionListener
    {
  private Point startingPoint, endingPoint, movingPoint;
  private Rect rectangle;
     private int x, y;

                 public void mousePressed(MouseEvent event)
                  {
                         //Needs to be filled
                   startingPoint = event.getPoint();
                  }

                 public void mouseReleased(MouseEvent event)
                  {
                         //Needs to be filled
                   endingPoint = event.getPoint();
                   x = endingPoint.x - startingPoint.x;
                   y = endingPoint.y - startingPoint.y;

                   if(endingPoint != null && x >= 0 && y >= 0)
                   {
                    Rect rectangle = new Rect(startingPoint.x, startingPoint.y, x, y, currentColor);
                    rectList.add(rectangle);
                   }

                   canvas.repaint();
                  }

                 public void mouseClicked(MouseEvent event) {}
                 public void mouseEntered(MouseEvent event) {}
                 public void mouseExited(MouseEvent event) {}
                 public void mouseDragged(MouseEvent event)
                   {
                        //Needs to be filled
                   movingPoint = event.getPoint();
                   x = movingPoint.x - startingPoint.x;
                   y = movingPoint.y - startingPoint.y;

                   canvas.repaint();
                   }

                 public void mouseMoved(MouseEvent event) {}

     } //end of PointListener

    private class ButtonListener implements ActionListener
    {
     public void actionPerformed(ActionEvent event)
     {
      rectList.trimToSize();
      rectList.remove(rectList.size() - 1);
      repaint();
     }
    }

    private class ColorListener implements ActionListener
    {
     public void actionPerformed(ActionEvent event)
     {
      switch(colorList.getSelectedIndex())
      {
       case 'r':
        currentColor = Color.red;
        break;
       case 'b':
        currentColor = Color.blue;
        break;
       case 'g':
        currentColor = Color.green;
        break;
       case 'o':
        currentColor = Color.orange;
        break;
       default:
        currentColor = Color.black;
      }
     }
    }
} // end of Whole Panel Class

Answer 1

rectList和newList都需要初始化

rectList = new ArrayList<Rect>();
newList = new ArrayList<Rect>();

这里还有另一个 NPE 来源 - 你是 shadowing变量colorList

JComboBox<String> colorList = new JComboBox<String>(colors);

应该是

colorList = new JComboBox<String>(colors);